Moral point: If we regard economic gain as only as good as the good it can do people, then economic gain (especially for an industry that enjoys significant tax favours) cannot be used to justify support for human rights abusers. Doubly so, when the support is so direct as to be one step back from providing British soldiers to actually do the repressing.
Pragmatic point 1: it’s naive to arm unstable (any dictatorship is unstable) regimes with reasonably advanced weapon systems. You never know when they may be turned on you. (Yes, yes I know we’re not sending state-of-the-art tech, but we’re considerably hardening an unstable potentially hostile military).
Pragmatic point 2: arming a repressive regime does your image no good with the repressed citizens. Again, it is unknown, but likely that at some stage their favourable view of us would be useful. Having fresh memories of fighting oppressors armed with British weapons (oppressive regimes don’t always bother to change the markings on planes and tanks, and records have a habit of leaking) won’t make them like us.